Question

why are inert gases placed in zero group?

Answer 1

Inert gas elements have their outer most shell completed or full. So they have no tendency to enter into any chemical reaction by giving or receiving electrons. Hence, inert gases are chemically inert and placed in zero group (Group-viii) in the periodic table. Inert gases with zero valency.

Answer 2

Inert gases (“noble gases”) already have their full complement of electrons in their outer shell. They do not need to bond to fill their outer shell of electrons. So, they are most stable atoms in the periodic table and are found as individual atoms in nature. The smallest (and the most stable) is Helium (He). You will not find ANY helium compounds. However, it is possible to react certain very reactive atoms with the larger noble gases, e.g. Xe and Kr. So you can form XeF2, XeF4 and XeF6 and XeO3. Radon forms even more of these compounds since its outer shell is a LONG way from the nucleus.

It therefore makes a lot of sense to group all of these gases in “group zero